Micro-electro Mechanical Systems (MEMS) switches are devices that use mechanical movement to achieve a short circuit or an open circuit in the RF transmissions line. RF MEMS switches are specific micromechanical switches that are designed to operate at RF-to-millimetre-wave frequencies (0.1 to 100 GHz) and form the basic building blocks in the RF communication system. The forces required for the mechanical movement can be obtained, for example, but not exclusively using electrostatic, magneto static, piezoelectric, or thermal designs.
The advantages of MEMS switches over p-i-n-diode or FET switches are:                Near-Zero Power Consumption: Electrostatic actuation does not consume any current, leading to very low power dissipation (10-100 nJ per switching cycle).        Very High Isolation: RF MEMS series switches are fabricated with air gaps, and therefore, have very low off-state capacitances (2-4 fF) resulting in excellent isolation at 0.1-40 GHz.        Very Low Insertion Loss: RF MEMS series and shunt switches have an insertion loss of −0.1 dB up to 40 GHz.        Intermodulation Products: MEMS switches are very linear devices and, therefore, result in very low intermodulation products. Their performance is around 30 dB better than p-i-n or FET switches.        Very Low Cost: RF MEMS switches are fabricated using surface (or bulk) micromachining techniques and can be built on quartz, Pyrex; low temperature cofired ceramic (LTCC), mechanical-grade high-resistivity silicon, or GaAs substrates.        
MEMS switched can be categorised as follows:                RF circuit configuration—series or parallel.        Mechanical structure—Cantilever or Air-bridge.        Form of Contact—Capacitive (metal-insulator-metal) Resistive (metal-metal).        
FIGS. 1 and 2 show a typical MEMS capacitive switch 63 which consists of a thin metallic bridge 65 suspended over the transmission line 67 covered by dielectric film 69. The MEMS capacitive switch can be integrated in a coplanar waveguide (CPW) or in a Microstrip topology. Conventional capacitive switches have a layer of dielectric between the two metal layers (bridge and t-line).
In a CPW configuration, the anchors of the MEMS switch are connected to the CPW ground planes. As seen in FIG. 2, when a DC voltage is applied between the MEMS bridge and the microwave line there is an electrostatic (or other) force that causes the MEMS Bridge to deform on the dielectric layer, increasing the bridge capacitance by a factor of 30-100. This capacitance connects the t-line to the ground and acts a short circuit at microwave frequencies, resulting in a reflective switch. When the bias voltage is removed, the MEMS switch returns to its original position due to the restoring spring forces of the bridge.
RF MEMS switches are used in reconfigurable networks, antennas and subsystems because they have very low insertion loss and high Q up to 120 GHz. In addition, they can be integrated on low dielectric-constant substrates used in high performance tuneable filters, high efficiency antennas, and low loss matching networks.
RF MEMS switches offer very low loss switching and can be controlled using 10- to 120 kΩ resistive lines. This means that the bias network for RF MEMS switches will not interfere and degrade antenna radiation patterns. The Bias network will not consume any power and this is important for large antenna arrays.
Typical MEMS switches require typical pull down voltages of 10-60V (these can be significantly lower or higher depending on the exact configuration and material system). This is a large range to cover using a software controlled DC MEMS Switch.
It is an object of the present invention to provide an improved MEMS switch.